Enhancement: Ignore sync errors when operation not supported by local filesystem

The local backend has been modified to work with filesystems which doesn't support
the `sync` operation. This operation is normally used by restic to ensure that data
files are fully written to disk before continuing.

For these limited filesystems, saving a file in the backend would previously fail with
an "operation not supported" error. This error is now ignored, which means that e.g.
an SMB mount on macOS can now be used as storage location for a repository.

https://github.com/restic/restic/issues/2395
https://forum.restic.net/t/sync-errors-on-mac-over-smb/1859
